A common arrangement for providing heating and/or cooling for each of the units of multi-tenant buildings such as apartment houses or office structures employs a liquid heat transfer media, usually water, which is heated and/or cooled by a central furnace/cooler connected in a main flow circuit. Individual flow circuits for the individual units are connected to the main in parallel through individual thermostatically controlled shut-off valves. Thermostatic sensors for these valves are located in the individual units to automatically and independently control the flow of the heat transfer media through the circuits which condition their associated units.
It is often desirable to measure the thermal energy consumption of each unit for billing purposes and a variety of systems have been proposed for measuring the energy consumption of each of a number of flow circuits leading off the common main. These systems generally employ temperature sensors connected to the input and output of each circuit, to measure the temperature change over the circuit, a flow meter for each circuit to measure the rate of flow volume through the circuit and an integrator to generate a signal proportional to the integral of the varying temperature differential and flow for each circuit over a period of time. For example, U.S. Pat. Nos. 759,306; 1,903,717; 2,359,767; 3,014,370; 3,216,248; 3,301,049; 3,593,578; 3,631,717; 3,842,669; 3,979,952; 4,048,852; 4,049,044; and 4,157,034 disclose systems employing integrating flow meters for individual circuits and to computers for servicing multiple flow circuits to determine the energy consumption in each unit by the time integration of varying temperature differential and flow signals.
These systems are relatively complex and require a substantial number of sensors and extensive wiring. Their costs may represent a substantial portion of the total cost of the heating and cooling apparatus in original construction and when a computerized system is to be retrofitted to existing construction so that the individual units can be metered, as when a commonly owned apartment building is converted into individually owned condominiums, the metering cost is often prohibitive.